The use of carbon dioxide in enhanced oilfield recovery (EOR) techniques has resulted in the production of light hydrocarbon gas mixtures which have a high CO.sub.2 concentration. Consequently, a need has arisen to separate the hydrocarbon products and to recover the CO.sub.2 for EOR reinjection.
Based on volatility differences, the separation of methane from CO.sub.2 should be relatively easy. At typical demethanizer conditions, the relative volatility of methane: carbon dioxide is 5:1. However, CO.sub.2 co-exists in vapor and solid phases in some regions of the column if the CO.sub.2 concentration is sufficiently high. Solid CO.sub.2 can plug the column.
The distillation of ethane from CO.sub.2 is also limited by the formation of an azeotrope at an approximate composition of 67 percent CO.sub.2 and 33 percent ethane. In addition, the azeotrope does not significantly change in composition with different operating pressures. Similarly propane is difficult to separate from CO.sub.2 because the relative volatility difference is small. Thus, solvents are often used to enhance separation efficiency.
One distillation method known as the Ryan/Holmes process is disclosed in U.S. Pat. No. 4,318,723 to Holmes et al. This patent describes the distillation of acid gases from methane using a non-polar addition such as C.sub.2 -C.sub.5 alkanes which are said to prevent solids formation in the distillation column. For additional discussion of the Ryan/Holmes process, see Holmes et al., Hydrocarbon Processing, May 1982, pp. 131-136; and Oil and Gas Journal, Jun. 27, 1983, pp. 85-91.
U.S. Pat. No. 4,383,842 to O'Brian describes a process similar to the above Ryan/Holmes process. By increasing the amount of non-polar additive used, an upper portion of a distillation column is said to be operated at a temperature above the triple point of carbon dioxide while still effecting separation of methane from CO.sub.2.
U.S. Pat. No. 4,428,759 to Ryan et al. describes a Ryan/Holmes process wherein a non-polar, solidspreventing additive is taken from a bottoms product of a depropanizer distillation column.
Ryan et al., Chemical Engineering Progress, pp. 53-56, October, 1984, describes a four-column Ryan/Holmes process, wherein CO.sub.2 is split from ethane in a first column and the overhead CO.sub.2 product is recovered as a liquid product in a second column. The second column overhead containing both CO.sub.2 and methane is directed to a demethanizer column. A solids-preventing additive is used to enhance the methane and ethane separations and is recovered in a fourth column. See also McCann et al., "The Mitchell Alvard South CO.sub.2 Plants", "Paper 74A, A.I.Ch.E. 1987 Spring National Meeting, Houston, Tex. Mar. 29-Apr. 2, 1987.
A schematic flow sheet of a four-column, commercially installed Ryan/Holmes process adapted from McCann et al., is seen in FIG. 1. Briefly, a hydrocarbon gas mixture made up primarily of CO.sub.2 and methane is directed in a line 100 to a CO.sub.2 stripping column 102. C.sub.1-2 gases are distilled, in the presence of a lean oil additive introduced from stream 104, into overhead stream 106. C.sub.3+ hydrocarbons are removed as bottoms stream 108. The overhead stream 106 is partially condensed, and the remaining vapor is compressed into stream 110 and fed to CO.sub.2 recovery tower 112. A methane rich gas stream 114 containing on the order of 29 mole percent CO.sub.2 is taken overhead and partially condensed and refluxed. A CO.sub.2 rich bottoms stream 116 is produced. Methane rich vapor stream 118 is introduced to a demethanizer tower 120. A lean oil additive stream 122 of butane and heavier alkanes is introduced in the reflux to demethanizer 120 to inhibit solids formation. A vapor stream 124 is removed as a fuel gas product. The bottoms from demethanizer 220 are withdrawn via stream 126 for reflux and/or recycle as previously mentioned. Bottoms product stream 108 from the CO.sub.2 stripping column 102 is fed to depropanizer tower 128 wherein C.sub.3 rich overhead gas is condensed and a portion of the liquid product is taken off as LPG product stream 130. A butane and heavier hydrocarbon rich bottoms stream 132 is withdrawn and recycled as needed in the stream 104. Excess lean oil from the streams 226 and 132 is taken off as NGL product stream 134.
U.S. Pat. No. 4,370,156 to Goddin, Jr. et al. describes a process for separating relatively pure fractions of methane and carbon dioxide from gas mixtures, wherein a lean oil is used as an absorbent and to prevent solids and azeotrope formation.
U.S. Pat. No. 4,595,404 to Ozero et al. describes a process for regeneration of a CO.sub.2 gaseous stream by low temperature distillation wherein the CO.sub.2 can be obtained as a liquid side draw from the distillation column.
U.S. Pat. No. 3,058,893 to Cahn et al. describes a distillation arrangement, wherein the separation process of two individual distillation columns is combined into a single distillation column.
U.S. Pat. No. 4,419,188 to McCall describes an extractive distillation process wherein the distillation columns are thermally coupled, that is, two or more columns share a condenser and/or reboiler which can be a discrete piece of equipment or a tray in one column or the other.